The present invention relates to concrete mixes utilizing silica fume, and is particularly useful in no slump concrete mixes using silica fume. Very fine grain pozzolans, such as silica fume, are used in concrete mixes in place of some of the cement, or in addition to it, to provide strength and/or to improve the impermeability and denseness of the concrete. The latter are desireable characteristics in concrete because they improve freeze-thaw resistance and resistance to chlorides and other aggressive agents which attack concrete.
Silica fume, when added to concrete by itself, has a high water demand, often requiring an additional pound of water for every pound of added silica fume in order to maintain constant slump and workability of the concrete. For wet cast or slumping concrete, it has been found necessary to reduce the water required to place the mix by using a high range water reducing agent in the mix in order to achieve an acceptable water-to-cement ratio. For no slump concrete, silica fume mixes using high range water reducers to reduce the water have not proved successful, particularly when such mixes are steam cured. High range water reducing agents, or superplasticizers, are expensive.
The use of diatomaceous earth and silica fume separately as pozzolans is well known. Diatomaceous earth is usually used in mass concrete applications for the purpose of reducing the heat of hydration.
Recognizing the essential physical characteristics of diatomaceous earth and silica fume is critical to this invention and to the successful use of both products. There are cellular particle pozzolans and non-cellular, solid particle pozzolans.
Silica fume is a very finely divided amorphous solid mineral pozzolan having a particle size of less than 5 microns and often a particle size in the neighborhood of 0.01 to 1.0 microns. It is generally believed that this ultra-fine size is necessary for providing tremendous surface area for reaction with cement/water hydration products. The ultra-fine, solid and hydrophobic silica fume particles tend to pack together, requiring either a substantial amount of water or, alternatively, a potent high range water reducing agent for wetting and dispersion. Since the silica fume particles are solid, they must rely on available water from the concrete matrix for continued hydration, or from water provided externally from a curing environment, either from mist or from submergence in water. The problem with dispersing silica fume with water only is that the water/cement ratio becomes unacceptably high. The problem with dispersing silica fume with potent high range (anionic) water reducing agents is that there is insufficient water for both the cement and the silica fume particles to hydrate when subjected to an air environment. The dilemma that has been facing the concrete industry is that silica fume concrete behaves acceptably well in a laboratory environment when cured in a mist room or when submerged in water. It does not perform well when subjected to an air curing environment during the first 10 to 28 days, which is usually the case in field use. In an air curing environment, a concrete mix of cement, silica fume, water and high range water reducing agents often performs poorly because the cement and the silica fume are competing for a limited and insufficient amount of hydration water. Unhydrated silica fume is known to cause micro-cracking in the micro-structural concrete matrix.
Diatomaceous earth is a cellular particle mineral pozzolan having an average size of greater than 5 microns and less than 20 microns. As a cellular particle, diatomaceous earth can absorb up to 2 times its dry weight in water. The cell structure of diatoms in diatomaceous earth is such that the water absorbed into its cells is tenaciously bound. This differs from other cellular materials such as cellulose which absorb water quickly, but give it up at an equally rapid rate. Diatomaceous earth has a large surface area for a given particle size because of its cellular make-up, as opposed to silica fume which is round, smooth and solid. In that diatomaceous earth can internally retain moisture, moist curing is not such a critical factor for concrete containing diatomaceous earth because the relatively large volume of water contained in the diatomaceous earth can feed the hydration of cement as well as the hydration of the diatomaceous earth. We believe that the rate at which the diatomaceous earth gives up its water is quite ideally suited to the rate at which all of the hydrating products in a concrete matrix can use the water for the hydrating process. Consequently, diatomaceous earth concrete performs well in an air-cured environment during the first 10 to 28 days, and thereafter, Without the hydrating and micro-cracking problems as experienced with the use of silica fume by itself. The limitation of diatomaceous earth as a pozzolan is that it can not, on a pound for pound basis, achieve the high strength and durability quality of moist cured silica fume concrete compositions.